In recent years, the performance of transmissive or emissive type displays such as LCD and OLED has increased significantly in metrics such as resolution, colour gamut capability and brightness, and decreased in cost such that they now form the large majority of the electronic displays market for most applications, both static and mobile, indoor and outdoor use. This has resulted in the retreat of reflective and transflective display types into niche applications for very high ambient illumination, and long battery life requirement applications. Even applications which until very recently a reflective display technology was preferred, such as outdoor signage, e-readers and smart wristwatches, are now largely being served by transmissive or emissive devices, due to their increased image quality capability. In these areas, and others in which a display device may be intended for use mainly in moderate ambient, or only occasionally high ambient situations, such as smartphones, tablets, automotive displays and notebook PCs, transmissive or emissive type displays may be modified to have improved performance in higher ambient lighting situations, with minimal impact on cost and dark room performance. Such modifications include the use of anti-reflection or anti-glare films to reduce reflections from the front surface of the display, and a circular front polariser to absorb reflection of ambient light from within the display. Circular polarisers are particularly effective at removing internal reflections and as result are used in displays such as LCDs in which higher dark room contrast may be obtained using standard linear polarisers (also sometimes referred to as plane polarizers), and OLEDs which do not use polarised light and therefore an emitted brightness loss is incurred.
The dominant LCD display technology for high resolution, narrow-bezel, wide-viewing angle applications such as smartphones and tablets, the Fringe-Field Switching (FFS) mode, is not conventionally compatible with circular polarisers, as at all voltage conditions, including zero, they have a LC director orientation, and therefore optic axis, with a large component in the polarisation plane of on-axis light, so no black state is achievable. This is also true for other commonly used LC modes such as In-Plane Switching (IPS), Twisted Nematic (TN) and Electrically Controlled Birefringence (ECB). These LC modes rely on the use of linear polarisers having a transmissive axis aligned parallel or orthogonal to the projection of the optic axis of the LC in the plane of the cell, in at least one of the display voltage states to produce a particular transmission condition.
Many methods have been developed to add a degree of controlled reflection of ambient light to FFS type displays in order to improve sunlight readability, by the inclusion of a reflective portion in each pixel with voltage controlled reflectivity. This can take the form of increased reflectivity of display electronics portions within the pixel (SID'07 digest, p706, BOE Hydis), or of a mirrored pixel potion used in conjunction with another pixel structure modification such as patterned or additional counter substrate electrodes (SID'07 digest, p1258, Hitachi), (US2014 0204325A1, Semiconductor Energy Ltd), an in-cell reflective polariser (App. Phys Lett, 92, 0501109, 2008, Ge et al). a variable cell gap thickness (SID'07 digest, p1651, Hitachi), (Optics Express, 19, p8085, 2011, Lim at el), patterned LC alignment (US 2010 0110351A1, Chi Mei), (SID'10 digest p1333, HannStar), (SID'10 Digest p1783, LG), but none of these methods reduce uncontrolled reflections from within the display stack, either separately or as part of the method of controlled reflection, and all add cost and manufacturing complexity due the requirement for additional spatially patterned layers to be deposited.
The publication (Applied Physics Letters 87, 011108, 2005, Song et al) describes a transflective FFS type display without additional spatially patterned layers, using two internal quarter wave plates (QWPs) on the lower substrate only, and active matrix electronics on the top (viewer-side) substrate. This structure allows controlled reflections from the reflective portion of the lower substrate only, but due to the two additional QWP layers being adjacent in the transmissive portion of the pixel, does not reduce uncontrolled reflections from any internal interfaces in this area. The second internal QWP serves solely to allow the first QWP to be deposited uniformly over the whole display area without affecting the optics of the transmissive pixel portion. The proposed structure would also add significant uncontrolled reflections from the active matrix electronics on the viewer side, which in a more standard arrangement would at least be attenuated by the reflected light having to pass twice through the colour filter layer.